Bioclimatic analysis of a site plan for Avignon-Confluences


with ChartierDalix architects

The ChartierDalix agency has realized a mass plan for the development of the Avignon-Confluences site, near the TGV station of Avignon, within the framework of a competition. The objective of this bioclimatic analysis is to study the exposure of the future constructions to the Mistral wind, the sunshine of the facades and to propose recommendations for the improvement of the interior and exterior thermal comfort. The devices recommended in this study aim to maximize the use of passive systems for heating, cooling, solar protection and natural lighting.

ChartierDalix chose a staggered morphology, which allows for natural ventilation of the different buildings. This choice also homogenizes the temperature near the ground and limits the urban heat island phenomenon that is accentuated in "canyon streets". Here, the wide ventilation between the buildings and the large portion of visible sky (Sky View Factor) favor night cooling and thus improve thermal comfort in hot weather. However, the study showed that this fragmented morphology and the location of the site outside the urban area (class II) aggravate the exposure of the outdoor spaces to the mistral, hence the need to install numerous windbreaks.

For this study, the entire future district was modeled. Simulations of sunshine for different periods using a heliodon and a virtual wind tunnel were carried out.

Site of the project of Avignon-Confluences Site of the project of Avignon-Confluences

Blender modeling of the layout Modeling of future constructions and existing hedges

In order to know the existing solar masks in the future development, different simulations have been performed with a heliodon. They show the effect of high-rise buildings on the shading of the streets and surrounding buildings. As it is, the building facades benefit little from the masks created in summer, because the morphology is not dense enough. A denser morphology would have allowed for mutual protection of facades, as with traditional urban planning in hot, dry climates.

Heliodon for winter solstice Shadows created by the constructions at the winter solstice (December 21)

Heliodon for equinoxes Shadows created by the constructions at the equinoxes (March 21 and September 21)

Heliodon for summer solstice Shadows created by the constructions at the summer solstice (June 21)

The western orientation is the most unfavorable for the rest rooms, and especially for the bedrooms. Indeed, the contributions by the thermal bridges and especially by the openings to the west from 14 h aggravate the thermal discomfort inside which is already degraded by the high temperature of the early afternoon.

The simulations show that the thermal discomfort inside the house is worsened by the high temperature of the early afternoon. They also show that the western facades are very exposed during the summer period. The openings to the west not protected by external protections (sunshades, shutters, blinds, etc.) can generate very unfavorable solar gains.

Heliodon for the western facades on June 21 Highlighting of the important exposure of the western facades at the summer solstice (June 21)

In order to better understand the action of the mistral on and between buildings, the model was tested in a virtual wind tunnel with a north wind with a slight north-northwest component. This wind tunnel allows to understand the pressure and depression zones but remains a coarse model that does not allow to understand the pressure and depression zones. This wind tunnel allows us to understand the pressure and depression zones but remains a coarse model that does not replace a CFD (Computational Fluid Dynamics) simulation.

Virtual Wind tunnel in Blender Test air flow acceleration between buildings and identify areas of excess pressure

Analysis of Venturi effect between two buildings Analysis of the Venturi effect between two buildings (light blue indicates slight acceleration of the air flow).

Numerical model for wind Virtual wind tunnel analysis of mistral effects: color reflects airflow acceleration

The bioclimatic study of this master plan highlighted the need to install numerous windbreaks in order to preserve the outdoor thermal comfort of pedestrians from the effects of the mistral.

The best windbreaks should not be opaque: they should have a geometric porosity of 0.2 (Φ = 0.2, or 20% void). The idea is to create turbulence in the wind flow, so some porosity improves the performance of a windbreak.

Agricultural netting, dense hedgerows in series and planted embankments work remarkably well. The advantage of a porous windbreak is that it attenuates the violent gusts of the Mistral wind but remains relatively permeable for low speeds: it thus allows for natural ventilation in summer.

Masonry windbreak of the Crau plain Example of a masonry windbreak in the Crau plain (source : Alain Guyot & Jacques Gandemer)

Agricultural windbreaks in series Principle of windbreaks in series (source: Alain Guyot)